As far as casting speed goes, Cure cast time merits and Cure Clogs will already have you near the cap when you couple it along with either Fast Cast from /RDM or Light Arts from /SCH. Cure Clogs have gotten rather expensive nowadays (at least on my server) but they are worth it, IMO.
This is dangerously misleading by the way Fyn, I don't know if you're unaware of how cast speed works or if this was just poorly worded but Cure Cast merits, Cure Clogs, and /RDM won't put you anywhere near the casting speed cap (assuming there even is one).
Of course it's basically impossible to test for certain, but after years of experimenting with various cast speed pieces I've come to the conclusion that there are several different cast speed stats, not just one. "Fast Cast" is one, and "Cure Cast" and "Healing Cast" may also be separate stats as well. I think they compound. So for example, if you have 30% fast cast and 30% cure cast, you'd end up with 70% * 70% = 49% total cast speed.
I've heard rumors of a 50% cap on fast cast... this may be true, but if it is, then it only applies to the vanilla "fast cast" and not to the other types. For example you could get 50% fast cast plus 20% cure cast, and you would end up with significantly more than a 50% total reduction in cast time for cures.
I've done some simple testing that, while it doesn't reveal the finer mechanics, does at least prove that I'm not just pulling your leg. (A lot of this is reposted from my previous posts on various forums.)
I did some testing first with only Light Arts (10%), Cure Clogs (15%), Pantaloons+2 (12%), and my merits (20%). The total if you add them up is 57%. So if there is indeed an overall cap of 50%, I should be well past it. I did multiple casts of various long spells, and they tended to finish around the 53% mark on the spell cast gauge.
Next, I added more equipment:
Orison Locket (5%)
Loquacious Earring (2%)
Winged Wand (2%)
Veela Cape (2%)
Ebur Bliaut (Healing Cast Time -3%)
Incantor Stone (2%)
This adds another 16%, giving me 23% fast cast, 35% cure cast, and 15% healing cast (73% total if you add them up). After doing this, my casts now finish around the 42% mark on the spell cast gauge. I realize that eyeballing the spell cast gauge is not exactly scientific, but the reduction was over 10%... it's hard to chalk that up to latency and/or human error, it's a pretty huge difference.
If my theory of compounding is correct, then the above setup would give me 0.77 * 0.65 * 0.85 = 0.425425, or 42.5% cast time, which matches exactly with my observation. If you do the same for the first test, you get 0.9 * 0.65 * 0.88 = 0.5148, which also almost exactly matches my observation.
My pet theory is that each of the three stats are separate, each compounds with the others, each has a cap of 50%, and there is likely a cap on the total cast speed of 80%. That's just what I'm inclined to believe, but it makes sense if you think about it. Haste was done exactly in this manner. There's a cap for haste from equipment for example, which is 25%, but by combining different kinds of haste it's possible to reduce delay by as much as 80% at which point you hit the hard cap on delay reduction.
P.S. Adding fast cast atma makes my casts finish even faster, well within the 30% range on the spell cast gauge. Celerity takes it even further, finishing the cast at around 25%. Don't be afraid to stack cast speed stats until the cows come home, WHM can cast stuff incredibly fast with all the gear that's available now. I feel like if I was casting much faster, the spell would finish before I started it.